What are deferred bounces (what does “Deferred” mean)?
Deferred emails are emails that have been temporarily refused delivery by the recipient’s mail server (or an intermediate server). When an email’s status is updated as Deferred, this does not indicate a permanent bounce (either hard or soft).
Instead, our system will automatically retry delivery of the message. Usually, email deferrals resolve on their own and the message is delivered later or permanently bounced, after which the status will update to Delivered, Soft bounce or Hard bounce.
What’s the difference between hard bounce, soft bounce and deferred?
Hard-bounced emails are permanently blocked from being delivered and should be added to your suppressions. These are emails that are undeliverable due to a nonexistent mailbox or because the recipient's mail server has permanently blocked delivery.
Soft-bounced emails are temporarily unable to be delivered. Soft bounces are caused by full mailboxes, the recipient’s mail server being temporarily unavailable, messages that are too large, and other temporary issues.
Deferred is a temporary status (available on Starter plans and above) that indicates an email couldn’t be delivered on the first attempt, but delivery will continue to be retried for a period of time. They may be deliverable if the temporary issue preventing delivery is resolved. If delivery isn’t successful during the retry window, the message will eventually result in a hard or soft bounce.
What’s happening when an email is deferred
When an email is deferred, its delivery has been delayed due to a temporary issue, and will be retried for a set period of time. A deferral is caused by:
The recipient mail server is busy, overloaded, or unreachable
The recipient’s inbox is full
Delivery is rejected due to security or policy checks, such as filters or greylisting applied by the recipient server, or the need for a secure connection
Missing or incorrect SPF/DKIM records or message authentication issues
Network/connection interruptions between servers
What to do if an email is deferred
Usually, deferrals will resolve on their own, and you won’t need to do anything. Occasionally, the deferral reason points to the issue being on the sender's side, or the problem repeats across many recipients. In this case, some investigation and troubleshooting are needed. (See the table of error codes and messages below.)
Immediate user action is recommended for:
Cases involving authentication/DNS errors (450/4.1.8, 451/4.7.0) or repeated 421/450/451 errors with patterns that suggest issues with reputation or sending rate
You may want to contact the recipient admin for:
Content filtering or relay restrictions (454/4.7.1, 459/4.7.1)
MX/DNS resolution issues (4.4.x, 4.1.2)
Recipient mailbox is full (452/4.2.2)
When to contact support
Before contacting support, we recommend that you wait for the deferral to resolve on its own—our system retries delivery automatically. And, if you receive any DNS/authentication errors, verify that your SPF/DKIM/DMARC/DNS records are correct.
We then recommend that you contact support if:
You receive repeated deferrals for many recipients at once. This could be a potential sending issue related to rate limits or IP reputation
You can’t resolve an authentication or DNS issue yourself and need assistance
To help us resolve your issue faster, when contacting support, please share the full, original bounce/deferral message, including the SMTP code and sub-code, plus the error message and ID of the email that caused the deferral. These often contain recipient-server-specific hints that help diagnose the cause faster.
Common deferral reasons, error codes and actions
In this table you’ll find the possible error messages and their likely causes. You can find a summary of the error on the Activity page, or view the full error via a webhook event (activity.deferred).
SMTP code/sub-code | Reason | Likely cause | Recommended action | What happens next |
SMTP 451/4.7.0 | Your message delivery has been temporarily delayed due to authentication issues | IP reputation issues, sender authentication (e.g SPF, DKIM) Or Server overload, security policy or general connectivity issues | Check SPF/DKIM/DMARC and sending IP reputation; review sending volume; contact support if you need assistance | Automatic retries; fix authentication issues to avoid repeated deferrals |
SMTP 451/4.7.0 | Your message delivery has been temporarily delayed due to secure connection requirements | Not able to establish secure connection due to TLS requirement on recipient server, Incorrect server configuration Or Recipient network or server infrastructure problems, Greylisting to temporarily reject messages from unknown senders, server overload | Check for secure connection; contact support if you need assistance | Automatic retries; ensure secure connection to avoid repeated deferrals |
SMTP 421/4.4.5 | Your message delivery has been temporarily delayed due to server connection limits | Server overload, excessive connections, network issues | No immediate action is required, these are usually resolved automatically | Automatic retries |
SMTP 421/4.7.2 | Your message delivery has been temporarily delayed due to unusual sending activity detection | Sending rate limits, unusual traffic patterns or spam-like from sending server, IP or domain reputation or Temporary server overload of recipient server | Pause or throttle your sending, check for spikes or compromised accounts, and contact support if needed. | Automatic retries; investigate if repeated |
SMTP 450/4.0.0 | Your message delivery has been temporarily delayed due to email delivery issues | Sending rate limits or Recipient server is temporarily unavailable, mailbox full, greylisting to temporarily reject emails from unknown senders, temporary problems with network | Check sending rate and monitor | Automatic retries; investigate if repeated. |
SMTP 450/4.1.8 | Your message delivery has been temporarily delayed due to sender domain verification issues. We'll retry automatically | Missing or Incorrect DNS Records, Domain Authentication Failure | Check your domain DNS records and authentication (SPF, DKIM, DMARC). Contact support if you need assistance | Automatic retries; fix auth to avoid further deferrals |
SMTP sub-code 4.4.1 | Your message delivery has been temporarily delayed due to SMTP connection issues | Recipient is experiencing network problems, server overload, firewall/security, misconfiguration on server | No immediate action; contact recipient admin if persistent | Automatic retries |
SMTP sub-code 4.3.0 | Your message delivery has been temporarily delayed due to network configuration issues | Recipient is experiencing temporary server overload or resource issues, temporary network connection issues, server misconfiguration | No immediate action; contact recipient admin if persistent | Automatic retries |
SMTP sub-code 4.1.2 | Your message delivery has been temporarily delayed due to host resolution issues | Temporary issue with recipient's email address or domain, blacklisting of outgoing mail server, network or firewall issues blocking communication | No immediate action; if persistent, recipient admin or DNS owner should investigate | Automatic retries |
SMTP sub-code 4.4.4 | Your message delivery has been temporarily delayed due to mail exchanger resolution issues | DNS problems resolving recipient MX, or temporary network outages | Usually no action on sender; contact recipient admin if persistent | Automatic retries |
SMTP 454/4.7.1 | Your message delivery has been temporarily delayed due to content filtering or relay access restrictions | The recipient mail server may be temporarily unavailable, network problems, overloaded server/insufficient disk space, authentication or configuration errors, firewall restrictions | Review message content/headers; contact recipient admin to allow relay or whitelist | Automatic retries; may need recipient admin action |
SMTP 457 | Your message delivery has been temporarily delayed due to greylisting | Recipient server intentionally delaying unknown senders | No action—greylisting expects retries | Automatic retries; typically accepted after a few retries |
SMTP 452/4.2.2 | Your message couldn't be delivered because the recipient's inbox is full. We'll retry automatically | Recipient mailbox quota reached | Ask recipient to free mailbox space or retry later | Automatic retries; may become permanent bounce if mailbox remains full |
Need more info?
Please feel free to contact us at support@mailersend.com and a member of our support team will gladly help you.
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